Method of electrolytically coating lead dioxide on the surface of various materials

ABSTRACT

Lead dioxide is coated on the surface of a base electrode by electrolyzing an acidic aqueous lead nitrate solution containing a small amount of copper nitrate at a temperature not lower than 70* C. and with a current density of anode not higher than 4 A/dm2.

United States Patent Kiyohara et an.

[451 June 6, 1972 [54] METHOD OF ELECTROLYTICALLY COATING LEAD DIOXIDE ON THE SURFACE OF VARIOUS MATERIALS [72] Inventors: Shinzo Kiyohara; Yasuichi Shibazaki, both of Yokohama, Japan lsomura Sangyo Kaisha, Ltd., Tokyo, Japan 22 Filed: Aug.2l,1969

21 Appl. No.: 852,074

[73] Assignee:

[58] Field of Search ..204/57 [56] References Cited UNITED STATES PATENTS 3,463,707 8/ 1969 Gibson et al ..204/57 3,213,004 10/1965 Schmidt ..204/57 Primary Examiner-John H. Mack Assistant Examiner-R. L. Andrews Attorney-Sughrue, Rothwell, Mion, Zinn & Macpeak 5 7] ABSTRACT Lead dioxide is coated on the surface of a base electrode by electrolyzing an acidic aqueous lead nitrate solution containing a small amount of copper nitrate at a temperature not lower than 70 C. and with a current density of anode not higher than 4 A/dm".

2 Claims, No Drawings METHOD OF ELECTROLY'I'ICALLY COATING LEAD DIOXIDE ON THE SURFACE OF VARIOUS MATERIALS The present invention relates to a method of galvanizing electrolytically lead dioxide coating on the surface of a base electrode consisting of various materials.

It is well known that a lead dioxide electrode having originally a high oxygen over voltage and hardly soluble characteristics in electrolytic'solution has failed to be useful industrially due to its insufficient mechanical strength of the deposited or plated lead dioxide even though lead dioxide itself is a hard and brittle material, and due to the difficulties encountered for the manufacturing of large lead dioxide electrode in such a commercial sizeas is normally used by an artificially made graphite electrode in the electro-chemical industry.

It is also a well known matter that a number of experiments have been carried out to deposit or galvanize electrolytically a lead dioxide coating on the surface of some base electrode, such as a tantalum-, or a nickel-screen, or on the surface of a graphite base electrode to improve the difficulties stipulated to enlarge lead dioxide electrode itself in the foregoing paragraph.

However, the inventors are of the opinion from their experiments that the life of a thus electrolytically deposited or plated lead dioxide coating on a certain base electrode in an acidic lead nitrate solution is greatly dependent upon the conditions under which the electrolysis is carried out.

Namely, due to complicated factors under which the electrolytic lating or coating is made on the surface of base electrode, for instance, such as the difference in the material of the base electrode and that of the electrolytic conditions used in the deposition of lead dioxide, the development of a coating having the excellent characteristic of difficult solubility in a salt solution and having a high over voltage for oxygen, has been unsuccessful.

The inventors have recently found and confirmed by X-ray difraction analysis of the layer of lead dioxide coating that one of the reasons for the decay or cleavage of this coating is attributable to the transition of amorphous lead dioxide electrolytically deposited into crystalline lead dioxides during the course of the applied electrolysis after the coating was prepared. 1 I

The present invention relates to the establishment of optimum electrolytic conditions under which a durable lead dioxide coating or plating can be made on the surface of a base electrode. I v

A detailed explanation of the conditions will be given in the following paragraphs:

An artificial graphite electrode as an anode of an electrolytic cell for the electrolysis of an alkali-chloride solution in the chlorine industry is the only available material for commercial use. However, the quality of the graphite itself is directly related to the current efficiency, the operating life of the cell, and to the purity of the produced chlorine gas.

As the electrode distance between the anode and the cathode is normally fixed by the structural characteristics of the electrolytic cell, any decay of the active surface of the graphite anode in the course of the progressing electrolysis will tend to increase the distance to the opposite cathode and it will cause a decrease in current efiiciency of the cell and an increase in power consumption per unit amount of chlorine. Frequent lowering of the anode will be necessary and the replacement of decayed anodes will appreciably decrease the operating efficiency of the factory.

Especially in case of sodium chloride electrolysis using a mercury cathode, the graphite anode will decay and drop physically due to the porosity and the graphite powder disintegrates the sodium amalgam to produce gaseous hydrogen and it is mixed in the chlorine gas. This is one of the reasons an explosive gas mixture of hydrogen and chlorine can be obtained.

A platinum anode is preferable for such an anode having the nature of only a slight solubility in a salt solution and having a high oxygen over voltage, but is is too expensive due to the shortage of supply, thus the application of platinum anode is limited to the anode for a high grade of oxidation in the production of perchlorate, periodate, and persulphate which needs an extremely high oxygen over voltage.

Although the oxygen over voltage is a little lower than platinum, a lead dioxide anode is favored due to its abundance and easiness to produce by the electrolysis of a lead salt solution, and it has a reasonably difficult solubility comparable with platinum.

Electrolytically deposited lead dioxide is resistant to halogen ions, nitrate ion, and sulphate ion, and therefore it is quite suitable as an anode for the electrolysis of these salt solutions. By the application of a lead dioxide anode, the adjustment of the distance between the anode and the cathode of the cell will become negligible and it will contribute to getting higher current efiiciency as well as higher productivity of the cell. Especially, the merit of using a lead dioxide anode can be found in getting pure chlorine gas'from the anode, because a graphite anode will give carbon dioxide inevitably mixed with chlorine gas together with a mixing of hydrogen due to the catalytic disintegration of sodium amalgam by the dropping powder of graphite, as was already mentioned in the foregoing paragraph.

lt was already reported in Journal Electro-Chemical Soc. Japan 552 i959) 27, that electrolytically deposited lead dioxide had three configurations of amorphous, crystalline alphatype (rhombic system), and beta-type (tetragonal system), while lead dioxide made from acidic lead nitrate solution consists of alphaand beta-type lead dioxide as a mixture, but lead dioxide deposited from alkaline lead salt solution contains alpha-type lead dioxide only. However, no information has been available to explain the mixing rate of these three configurations as well as the producing conditions for the respec' tive types of lead dioxide by electrolysis.

The inventors tried for the first time to, check this matter by a phase analysis using an X-ray defraction method for the deposited lead dioxide which was prepared under following conditions:

Electrolytic cell: as reported in Journal Electro- Chemical Soc. Japan 59 1968) 36 Electrolytic solution: Pb(NO;,)'=mol/liter, Cu(NO 0.l mol/liter pH 4.2

Bath temperature: 30-70 C Current density: l'l0 A/dm? at the anode Base metal of anode: Platinum plate Lead dioxide used in Electrolytically deposited the experiments: lead dioxide on platinum plate is taken off, and it was washed with distilled water and was dried.

2. Generally speaking, a higher content of beta lead dioxide 7 is obtained the higher the temperature of electrolysis, ex cept in the case the electrolysis temperature is low and the current density is also low. At a lower temperature, the variation of the content of beta lead dioxide is not remarkable.

3. The content of amorphous lead dioxide is higher when the bath temperature is low and the current density is high. By life tests for the electrolysis of a halide solution by using electrodes coated with lead dioxide untilsome remarkable decay or cracking of the coated layer was noticed, the following results are reported:

a. An electrode coated with lead dioxide at a lower temperature and at a higher current density during deposition had a life of l 6 months of continuous use.

b. An electrode coated with lead dioxide at a higher temcathode Anode: I mm from the top of No.1 perature and at a lower current density during deposition 4 electrodes shown in hadacompmfiveylongwife- $:.:::.:.:::'.:.".:.":.;:':::2.

This will indicate that amorphous lead dioxide tends to change 3 A/dmr its configuration into crystalline forms during the period of 5 Upper part oftheele trode electrolysis in the halide solution and the cleavage or decay is amf fifg' ai f attributable to the change of the specific volume between Weight Loss: By decreased weight afier the these configurations. (refer to (3) of foregoing paragraph). As tests. the result, it is quite evident that the determination of conditions under which a lesser content of an amorphous lead diox- The results ofthe tests are given in Tab? TABLE 2 Weight of electrode Weight Before After Decreased Current loss per test test weight used, A11 in Number in g. in g. in g. AII mgJAlI Remarks 137. 9253 137. 7370 0. 1873 356 0.625 Pin holes found. 140.6917 140. 5835 0.1082 485 0. 223 Crnckings found. 139. 4512 139.4113 0.0399 503 0.0793 Few pin holes. 140. 9234 140. 0232 0. 0002 512 0.000 N0 defects.

ide containing coating can be made, is the most important EXAMPLEZ matter. For instance, lead dioxide coating made with a current Accordtn to .la anese Pat. Nos. 450 342 and 466 747 b densny of l Ald'mg at the anode and at a bath temperature of the inventofs holl ow units having an outer diameter of 4% 70 C. showed a minimum content of amorphous lead dioxide mm an innerdiameter of 34 mm and a length of l 00 mm g m the comma It already reported by the Inventors m tained by molding electrolyzed lead dioxide containingasmall Journal Electro-Chemical Soc. Japan 91 l (1962) that a 0.5 amount of a thermoplastic synthetic resin as an adhesive in a 1.35 mm thickness of a lead dioxide coating could be made sealed pressure proof mold under pressure and at elevated s 2 at 8 0 65 ham i f A/dm. current temperature were connected under prmure and at elevated dens1ty at the anode from an acidic lead nitrate solution upon 30 temperature to 730 mm including a sehfi sphcfical end pic the surface of or grflphlte base q' The inside of this electrode was washed with water and was Thus, the present nventton ts characterized in that an aqueelectrolytically galvanized with a silver coating 10 microns ous solution of acid c lead nitrate is electrolyzed by using a in thickness by a known pmqess Again it was washed with ham temperature 'E F than a a current denmy water and was galvanized with a copper coating of 20 microns ower than 4 A/dm order to the content of 35 from an acidic copper sulphate solution. The outside surface amorphous lead dloxlde? of this base electrode was covered still with a film of ther- EXAMPLE 1 moplastic resin during hot-press processing, it was mechanically removed and cleaned to remove resinous material with The base electrode used in this experiment was made in ac- 40 some solvents. The treated outside surface of the base eleccordance to the process described in Japanese Pat. No. trode now underwent the lead dioxide coating process by the I 450,342 or US. Pat. Nos. 3,434,905 and 3,318,794 by the inll wing: i i ventors, which consists of a center copper bar of 3 mm in I diameter and 210 mm in length, and silver is galvanized on its Electrolyte: Pb( 190,), 1 mol/l, Cu( NO,),'0.l surface upon which lead dioxide powder mixed with a ther- Bath em mute f'g'g g moplastic resin was hot-pressed to make a lead dioxide base Current f 3 i at the anode electrode having a diameter of 8 mm and a coating length of Thickness of PhD, coating: 2 mm 180 mm with a semi-spherical end piece. v

The thus prepared base electrode now underwent another The 6 electrodes thus prepared were tested in a'commercial process for the electrolytic deposition oflead dioxide by using electrolytic bath using a 500 ampere cell without the use of a it a anode and a copper b r a athode in an a idi lead dtaphragm to produce sodium chlorate by the electrolysls of a nitrate solution underconstant currentdensity and other con- Saturated SOdlum chlol'lde l n I WhlCh 3 g/l Of sodlum' ditions given in Table 1. chromate was added at pH of 6.7 6.85. The current density TABLE 1 Conditions of electrolysis Electrolyte (urretnt Thilcllgrlreoss Spec. ensi y o resistance Ph(N0;)z, 011010.12, at anode, Temp, deposited? x10 Number moi/l. moi/l. p11 -Sp. gr. A./d1n. 0 mm. ohm-cm.

1.03 0.17 4. 2 1.32/20 0. 4.5 225:1. b 1.15 1.5 1.08 0.17 4. 2 1.32/20 C 9.0 25=l=2 1.85 8.0 1. 05 0. 15 4. 0 l 28/25 C 5. 0 70=l=2 1. 75 5. 3 1. 05 0. 15 4. 0 1 28l25 C 3. 5 77i2 1.80 3. 0

The thus coated lead dioxide layers can only be checked at the lead dioxide anode was 1 2.3 13.8 A/dm, a bath voltthe the stability by using it in a practical manner in the elecage of 3.7 3.9 volt with a cont nuous and total power supply trolysis of salt solutions. The inventors employed following process to check the life of the coatings in a comparatively All Of these electrodes showed 110 defect after the electrolyshort period. sis and no lead ion was identified in the solution after the electrolysis and the color of solution on completion was a thin yellow and clear. Elemoxyt'c sdunon' i gf g izg f fi 'ii The present industrial process to make sodium chlorate uses Bath temperature 40 1 2 C a graphite anode which is liable to give a dark mother solution Current: ampere after the electrolysis due to decayed carbon powder coming 5a diamem 2 0 mm in from the anode, and it will take expensive and big installationssse length. normal st'eel jar as to get a clear mother solution as well as higher labor costs to recover pure end products.

This invention seems to give a reliable and reasonable process for the electrolytic production of various useful materials on an industrial scale.

What is claimed is:

1. In a method of electrodeposition of a coating of lead dioxide on the surface of a base electrode which comprises electrolyzing an acidic lead nitrate solution containing a small amount of copper nitrate, the improvement which comprises 

2. The method of claim 1, wherein the current density at the anode ranges from 3 to 4 A/dm2. 